Shell Rock River IA 02-SHL-784

This evaluated biological assessment was based on data collected in 2007 collected by the IDNR Fisheries Bureau.  A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data.  The biological metrics are based on the numbers and types of fish species collected in the stream sampling reach.  The biological metrics were combined to make a fish community index of biotic integrity (FIBI).  The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).  The 2007 FIBI score was 70 (good).  The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the FIBI score with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008.  The riffle/non-riffle habitat FIBI BICs for this ecoregion are 65/44.  This assessment is considered evaluated because the drainage area (1454 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.  Even though this site passed the FIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the site used for the assessment doesn’t fall in the calibrated watershed size.

The fish consumption uses remain assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring in 2011.  The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes.  The fish contaminant data generated from the 2011 RAFT sampling conducted on the Shell Rock River near Clarksville showed that levels of mercury were sufficiently high for concern and thus justified issuance of a one meal/week fish consumption advisory in 2013 that extends from the mouth of the Shell Rock River to its confluence with the Winnebago River near Rockford in Floyd County.  Results from the 2011 sampling showed that the level of mercury in the sample of walleye muscle tissue (0.75 ppm) far exceeded the 1 meal/week trigger level (0.30 ppm) as defined in Iowa’s revised (2007) fish consumption advisory protocol.  According to the IDNR/IDPH fish advisory protocol, the single occurrence of contaminant above an advisory trigger level does not typically result in issuance of an advisory.  Such an advisory is issued only after follow-up monitoring confirms that contaminant levels exceed the advisory trigger level.  Because, however, the level of mercury in the 2011 sample of walleye was well above the one meal/week advisory trigger, a one meal/week consumption advisory was issued.  Follow-up monitoring will be conducted to better define the level of mercury in Shell Rock River fish and to provide information for any changes needed to the consumption advisory.  Because a one meal per week fish consumption advisory was issued for this waterbody, the fish consumption uses are assessed as “partially supported.”